The present invention relates generally to drilling fluid recovery and, more specifically, to a system for preventing fluids from being spilled when casing is being finished for a well bore.
The process of drilling subterranean wells to recover oil and gas from reservoirs, consists of boring a hole in the earth down to the petroleum accumulation in the reservoir, and installing pipe from the reservoir to the surface. Casing is a protective pipe liner within the well bore that is cemented in place to insure a pressure-tight connection to the oil and gas reservoir. The casing can be run from the rig floor as it is lowered into the well bore.
When running casing, drilling fluid is added to each section as it is run into the well. This procedure is necessary to prevent the casing from collapsing due to high pressures within the well bore. The drilling fluid acts as a lubricant which facilitates lowering the casing within the well bore.
Drilling fluid, or drilling mud, is very important to the rotary drilling process. Drilling and completion fluids which include fluids such as weighted mud, oil-based fluids, water-based muds and the like are often quite expensive and may frequently cost more than one million dollars per well. It is basically a mixture of water, clay, and special minerals and chemicals and performs many important functions. For example, drilling fluid exerts pressure inside the hole keeping fluids that may be in the formations from entering the hole and perhaps blowing out to the surface. In addition, pressure in the hole forces solid particles of clay in the mud to adhere to the sides of the hole as the drilling fluid circulates upward on its way to the surface. The solids form a thin, impermeable cake on the walls of the hole. If discharged drilling fluids can be hazardous to the environment.
The normal sequence for running casing involves suspending the casing from a top drive or non-top drive (conventional rotary rig) and lowering the casing into the well bore, filling each joint of casing with drilling fluid. The filling of each joint or stand of casing as it is run into the hole is the fill-up process. Lowering the casing into the well bore is facilitated by alternately engaging and disengaging elevator slips and spider slips with the casing string in a stepwise fashion, facilitating the connection of an additional stand of casing to the top of the casing string as it is run into the hole.
Circulation of the fluid is sometimes necessary if resistance is encountered as the casing is lowered into the well bore, preventing the running of the casing string into the hole. This resistance to running the casing into the hole may be due to such factors as drill cuttings, mud cake, or surface tension formed or trapped within the annulus between the well bore and the outside diameter of the casing, or caving of the well bore among other factors. In order to circulate the drilling fluid, the top of the casing must be sealed so that the casing may be pressurized with drilling fluid. Since the casing is under pressure the integrity of the seal is critical to safe operation, and to minimize the loss of expensive drilling fluid. Once the obstruction is removed the casing may be run into the hole as before.
Once the casing reaches the bottom, circulating of the drilling fluid is again necessary to test the surface piping system, to condition the drilling fluid in the hole, and to flush out wall cake and cuttings from the hole. Circulating is continued until at least an amount of drilling fluid equal to the volume of the inside diameter of the casing has been displaced from the casing and well bore. After the drilling fluid has been adequately circulated, the casing may be cemented in place.
After the casing has been run to the desired depth it may be cemented within the well bore. The purpose of cementing the casing is to seal the casing to the well bore formation. In order to cement the casing within the well bore, the assembly to fill and circulate drilling fluid is generally removed from the drilling rig and a cementing head apparatus installed. A special cementing head or plug container is installed on the top portion of the casing being held in place by the elevator. Since the casing and well bore are full of drilling fluid, it is first necessary to inject a spacer fluid to segregated the drilling fluid from the cement to follow. The cementing plugs are used to wipe the inside diameter of the casing and serves to separate the drilling fluid from the cement, as the cement is carried down the casing string. Once the calculated volume of cement required to fill the annulus has been pumped, the top plug is released from the cementing head. Drilling fluid or some other suitable fluid is then pumped in behind the top plug, thus transporting both plugs and the cement contained between the plugs to an apparatus at the bottom of the casing known as a float collar. Once the bottom plug seals the bottom of the casing, the pump pressure increases, which ruptures a diaphragm in the bottom of the plug. This allows the calculated amount of cement to flow from the inside diameter of the casing to a certain level within the annulus being cemented. The annulus is the space within the well bore between the ID of the well bore and the OD of the casing string. When the top plug comes in contact with the bottom plug, pump pressure increases, which indicates that the cementing process has been completed. Once the pressure is lowered inside the casing, a special float collar check valve closes, which keeps cement from flowing from the outside diameter of the casing back into the inside diameter of the casing. At this point the casing is filled with drilling fluid.
After being run the casing must be cut and finished at an appropriate level to install rig equipment such as blow out preventers along with other equipment. However, because of earlier operations, the entire length of casing is typically filled with drilling fluid. Depending on conditions, the length of casing which is to be cut and removed may therefore over a 100-foot (27.4 meter) column of drilling fluid therein. The drilling fluid in this section must be properly drained before the casing is cut and removed.
Prior art systems for removal of the drilling fluid in the casing have consisted of cutting an opening in the casing with a casing cutter, using tarpolines and a pan in an attempt to contain the escaping column of drilling fluid in the casing to be removed. Unfortunately, the prior art systems have been slow (taking up to many hours to drain) and allowing drilling fluid to and escape into the environment creating potential environmental hazards, such as pollution. Additionally, loss of fluids can be costly as the fluids are expensive and must be replaced.
While certain novel features of this invention shown and described below are pointed out in the annexed claims, the invention is not intended to be limited to the details specified, since a person of ordinary skill in the relevant art will understand that various omissions, modifications, substitutions and changes in the forms and details of the device illustrated and in its operation may be made without departing in any way from the spirit of the present invention. No feature of the invention is critical or essential unless it is expressly stated as being “critical” or “essential.”